1. Field of the Invention
The present invention relates to a light scanning apparatus to be used in an image forming apparatus such as a printer, a facsimile machine, a copier, etc., and more particularly, to an apparatus to detect a laser beam, and to produce a laser beam detect signal that may be used to synchronize scanning operation of the image forming apparatus, which is capable of reducing performance degradation caused by the dimension deviations and the assembly deviations introduced in the fabricating and assembling process.
2. Description of the Related Art
Generally, a light scanning apparatus of an image forming apparatus such as a printer, a facsimile machine and a copier, uses a light source that generates a beam of light such as a laser beam to form an electrostatic latent image on a photosensitive body such as a photosensitive drum or photosensitive belt.
The light scanning apparatus forms the electrostatic latent image on the photosensitive body by converting the laser beam from the light source such as a semiconductor laser into a parallel ray of light in a predetermined size through a collimator lens, deflecting a direction of the laser beam at a light deflector rotating at high speed, and emitting the laser beam along a scanning line on the photosensitive body through a scanning lens such as an f-theta lens.
In order to precisely locate the starting location where the electrostatic latent image is to be first formed on the photosensitive body (i.e., the starting location of the laser beam scanning line), an apparatus to detect the laser beam at a certain predetermined location relative to the intended starting point of the scanning line is employed. The apparatus to detect the laser beam, generates a beam detect signal, which is used by the image forming apparatus to synchronize the timing of the laser beam firing, or the like, so that the scanning is started at the intended starting point.
FIG. 1 schematically shows a conventional light scanning apparatus 10 to form an electrostatic latent image on a photosensitive body.
Referring to FIG. 1, the light scanning apparatus 10 includes a semiconductor laser 1 to emit a laser beam 14, a collimator lens 2 arranged in correspondence to the semiconductor laser 1 to form the laser beam 14 into a parallel ray of light, and a slit 3 through which the laser beam 14 passed through the collimator lens 2 is converted into a predetermined form. The light scanning apparatus 10 also includes a cylindrical lens 4 through which the laser beam 14 passed through the slit 3 is transformed into a linear light, and a light deflector 5 to deflect a direction of the laser beam. The light deflector 5 includes a rotary polygon mirror 5a supported on a motor (not shown) to be rotatably driven at a given speed.
The light scanning apparatus 10 also includes an f-theta lens 6 that compensates for error included in the laser beam 14 deflected from the rotary polygon mirror 5a, thereby emitting the laser beam 14 to a photosensitive drum 20. The light scanning apparatus 10 includes a beam detect signal generating part 30 that generates a signal used by the image forming apparatus to synchronize the image formation location of the electrostatic latent image along a laser beam scanning line 20a correctly.
The beam detect signal generating part 30 includes a reflective mirror 8 secured on a spring 7 on a portion of an optical path of the laser beam 14 that would not interfere with the scanning of the laser beam 14 along the length of the laser beam scanning line 20a. The reflective mirror 8 deflects the laser beam 14 in the direction of a beam detect lens 9. The beam detect lens 9 is secured on a frame 13 to focus the laser beam 14 from the reflective mirror 8 onto a photo diode sensor 11. The photo diode sensor 11 upon detection of the laser beam 14 generates a beam detect signal. The photo diode sensor 11 may be assembled either in a printed circuit board 12 (PCB), which also supports the semiconductor laser 1 which is secured on the frame 13, or in a separate printed circuit board (not shown).
An operation of the conventional light scanning apparatus 10 will be described below. In accordance with input image signals, the laser beam 14 is emitted from the semiconductor laser 1, and converted into the parallel ray of light by the collimator lens 2. Then, after passing through the slit 3 that shapes the laser beam 14 in a predetermined form, the laser beam 14 is passed through the cylindrical lens 4, and then deflected by deflecting faces of the polygon mirror 5a which is rotated at relatively high speed by the motor (not shown).
Next, the laser beam 14 is made to selectively pass through the f-theta lens 6 to be converged on the photosensitive drum 20 in a form of a light spot, thereby scanning the scanning line 20a of a predetermined effective scanning width along the main scanning direction as shown in FIG. 1. At this time, the photosensitive drum 20 is driven to rotate in the sub-scanning direction by a driving motor (not shown). Accordingly, as a result of scanning movements of light spots in the main scanning direction and the rotation of the photosensitive drum 20 in the sub-scanning direction, a predetermined electrostatic latent image is formed on the photosensitive drum 20.
In order to start each of the scanning lines 20a at the correct starting point, the laser beams 14 deflected from the rotary polygon mirror 5a is detected at a predetermined location either prior to the start of or past the end of the effective scanning width of the laser beam scanning line 20a. The laser beam 14, which have passed through f-theta lens 6 is deflected by the reflective mirror 8 placed at the predetermined location in the main scanning direction toward the beam detect lens 9. When the laser beam 14 deflected by the reflective mirror 14 is received by the photo diode sensor 11, the photo diode sensor 11 in response thereto produces a beam sensed signal. The beam sensed signal itself may be taken as the beam detect signal, or, in the alternative, is converted into suitable voltage and/or current, by a beam detect signal generation circuit (not shown), which may be disposed on the same PCB 12, to generate the beam detect signal.
The beam detect signal so generated is input to a controller (not shown), which controls timings of both a scanning start and the image formation of the light spots on the photosensitive drum 20. The controller uses the beam detect signal in order to determine the proper location for the scanning start.
However, the conventional light scanning apparatus 10 operated as above has a rather complex structure in which the photo diode sensor 11 of the beam detect signal generating part 30 is secured on the frame 13 through the PCB 12 or on another PCB separately prepared for the photo diode sensor 11, while the beam detect lens 9 is directly secured on the frame 13. Accordingly, dimension and assemblage deviations or errors are frequently generated during a process of fabricating and assembling parts such as the beam detect lens 9, the frame 13, the photo diode sensor 11.
When the errors occur during the fabrication and assembling, a center of an optical axis of the beam detect lens 9 is hardly aligned with a detecting area of the photo diode sensor 11. Accordingly, as the laser beams 14 are irregularly incident on the photo diode sensor 11, a detection location of the laser beam varies, and as a result, a constant printing quality is not guaranteed.